The monitoring and control of hydrogen concentration in gaseous and liquid media is an important technological issue. Fields of application include, for instance, the analysis of gas composition on the fuel side of hydrogen-based fuel cells and the determination of dissolved hydrogen content in molten metals like aluminium. It is therefore desirable to develop simple, easily applicable, reliable and inexpensive sensors having high sensitivity and selectivity.
One concept of constructing hydrogen sensors for operation at elevated temperatures is to utilise a proton conducting solid electrolyte that compares the hydrogen partial pressure on the measuring side with a known and fixed hydrogen partial pressure on the reference side. The most appropriate proton conducting solid electrolytes are perovskites, with doped strontium cerate (SrCe0.95Yb0.05O3-d) and doped calcium zirconate (CaZr0.9In0.1O3-d) being applied most frequently. Under the relevant experimental conditions, these materials exhibit predominant proton conductance. Electrodes are formed by covering the surface of the electrolyte with a catalytically active and electronically conducting material, for instance platinum. If two electrodes on different areas of the same electrolyte body are brought into contact with two media of different hydrogen contents, i.e., p′H2 and p″H2, a hydrogen concentration cell is formed:
p′H2|proton conducting solid electrolyte|p′H2
The potential difference generated may be described in terms of the well known Nernst equation, where U is the electromotive force (emf), R is the universal gas constant, T is the absolute temperature, F is Faraday's constant, and p″H2 and p′H2 are the hydrogen partial pressures at the measuring electrode and the reference electrode, respectively:
  ⋃      =                  -                  RT                      2            ⁢            F                              ⁢      In      ⁢                        p                      H            ⁢                                                  ⁢            2                    ″                          P                      H            ⁢                                                  ⁢            2                    ′                    
Measuring the potential difference between the two electrodes and knowing the hydrogen partial pressure on the reference side, yields the unknown hydrogen partial pressure on the measuring side.
The incorporation of a hydrogen reference standard into the sensor unit constitutes a scientific and technological problem. Two different types of hydrogen sensors employing a solid electrolyte in conjunction with a hydrogen reference have thus far been reported.
The most straightforward approach consists in the utilisation of a gaseous hydrogen standard [T. Yajima, K. Koide, N. Fukatsu, T. Ohashi and H. Iwahara, Sensors and Actuators B 13-14, 697 (1993); T. Yajima, K. Koide, H. Takai, N. Fukatsu and H. Iwahara, Solid State Ionics 79, 333 (1995)]. This requires a cell design in which one side of the solid electrolyte is in contact with the medium to be analysed while the other side is continuously supplied with a reference gas mixture of known hydrogen partial pressure. A hydrogen analyser for use in molten aluminium, based on this principle and using CaZr0.9In0.1O3-d as the solid electrolyte, has been developed and commercialised. However, the use of a reference gas has been found awkward, and no breakthrough with this technology has been achieved.
In alternative approaches attempts have been made to fix the hydrogen partial pressure on the reference side by means of solid compounds or mixtures of solid compounds. The utilisation of hydrates like Ce2 (SO4)3.8H2O and AlPO4.0.34H2O as the reference in conjunction with SrCe0.95Yb0.05O3-d and CaZr0.9In0.1O3-d as the solid electrolyte has been reported [H. Iwahara, H. Uchida, T. Nagano and K. Koide, Denki Kagaku 57, 992 (1989); T. Yajima, K. Koide, K. Yamamoto and H. Iwahara, Denki Kagaku 58, 547 (1990); T. Yajima, H. Iwahara, K. Koide and K. Yamamoto, Sensors and Actuators B 5, 145 (1991)]. However, incorporation of hydrates fixes the water rather than the hydrogen partial pressure and, even though some response behaviour to hydrogen has been observed in a few cases, these sensors require calibration and their signal stability is insufficient for practical applications. The utilisation of a calcium/calcium hydride (Ca/CaH2) mixture as the reference in contact with SrCe0.95Yb0.05O3-δ as the solid electrolyte has been reported [M. Zheng and X. Zhen, Solid State Ionics 59, 167 (1993); M. Zheng and X. Zhen, Met. Trans. B 24, 789 (1993); M. Zheng and X. Chen, Solid State Ionics 70/71, 595 (1994)]. However, this combination was only found to work at comparatively low temperatures, i.e., below 600° C., and for relatively short times, i.e., a few hours, otherwise a continuous drift of the sensor signal towards zero was observed. The reason for the failure was identified to be the chemical instability of the electrolyte/reference interface. This causes a chemical reaction between the highly reducing reference material and the oxide-based solid electrolyte, which gradually converts the ion (proton) conductor into a mixed conductor and renders sensor readings impossible to interpret. Overall, no hydrogen sensor relying on a solid reference material has as yet proven to be viable in any practical application.